In general, a gas oil composition is produced by blending one or more types of base oils produced by subjecting a straight gas oil or straight kerosene, produced by atmospheric distillation of crude oil to hydrorefining or hydrodesulfurization. In particular, it is often the case that the blend ratio of the foregoing kerosene base oil and gas oil base oil is adjusted in order to ensure the cold flowability during a winter season. If necessary, the base oils are blended with additives such as cetane number improvers, detergents and cold flow improvers (see, for example, Non-Patent Document No. 1 below).
Lower sulfur and aromatic contents are regarded as leading to suppressing of the formation of harmful components such as NOx and PM in the exhaust gas from engines. From the view point of this, attention has been brought to fuels such as liquid fractions corresponding to naphtha, kerosene and gas oil, produced by subjecting a mixed gas containing mainly hydrogen and carbon monoxide produced from natural gas, coal, biomass or sludge (hereinafter may be often referred to as “synthetic gas”) to a Fischer-Tropsch (FT) reaction; hydrocarbon mixtures produced by hydrorefining or hydrocracking such liquid fractions; and hydrocarbon mixtures produced by hydrorefining or hydrocracking liquid fractions and FT wax produced through a Fischer-Tropsch reaction, as fuels contributing to environment load reduction.
However, since the FT reaction per se contains a wax formation process, the hydrotreated products of the FT reaction product are relatively large in the content of straight-chain saturated hydrocarbon (normal paraffins) compounds. It has been pointed out that in particular when heavy normal paraffin compounds are contained in the hydrotreated product, there is a possibility that they would deposit in the form of wax. Further, the FT synthetic base oil is a hydrocarbon mixture containing predominantly the aforesaid normal paraffins and saturated hydrocarbons having a side chain (isoparaffin) and thus is generally poor in oil solubility. Therefore, there is a possibility that additives that are dissolved in fuel oils such as gas oil, highly relying on their oil soluble groups (straight-chain alkyl groups or the like) would be hardly dissolved. Among such additives, there would be likely used conventional cold flow improvers (CFI) composed of an ethylene-vinyl acetate copolymer mixture due to the restriction on the solubility to fuel.
Patent Document No. 1 discloses in an example thereof a synthetic fuel containing only a gas oil fraction produced from an FT synthetic base oil. However, this gas oil is an extremely light fuel containing a kerosene fraction in a large amount because the document intends to solve a problem concerning low-temperature startability and thus a technique for improving low-temperature properties with a cold flow improver can not be selected. As the result, significant reductions in density, kinematic viscosity and volume calorific value can not be avoided, and furthermore it can not be denied that the reductions would result in significant deterioration in fuel consumption, seizure of injection pumps, cavitation damages and defects in high-temperature restartability.
That is, it is very difficult to design a high-quality fuel that can achieve at a high level the requirements sought for a gas oil composition having environment load reduction properties, excellent practical performances in a winter season and suppression of fuel consumption deterioration all together, and there exists no example or finding on the basis of studies of such a fuel satisfying sufficiently various properties required for fuel other than the foregoing and a practical process for producing the fuel.                (1) Patent Document No. 1: Japanese Patent Laid-Open Publication No. 2005-529213        (2) Non-Patent Document No. 1: Konishi Seiichi, “Nenryo Kogaku Gairon”, Shokabo Publishing Co., Ltd., March, 1991, pages 136 to 144        